来自：http://www.imooc.com/video/7871
推荐大家去学习这个视频，讲解的很不错。
慕课网提供了一个json网址可以用来学习：http://www.imooc.com/api/teacher?type=4&num=30。我们的任务就是建立一个listview，将json提供的一些参数，主要是name，picSmall，description显示出来，效果图如下： 
主要思路如下：listview中图片的加载，程序中使用了两种方式，一种是使用Thread类，一种是使用AsyncTask，为了提高listview滑动的效率，并节省流量，使用了LruCache类，更改加载图片的处理逻辑为：当滑动listview时不加载图片，停止滑动listview时加载界面可视范围内的图片。具体程序如下：
1 NewsBean.Java listview每一项的封装类
package com.example.imoocnews;
public class NewsBean {
private String newsIconUrl;//图片的网址即picSmall
private String newsTitle;//图片的标题即json中的name属性
private String newsContent;//图片的内容即json中的description
public NewsBean(String newsIconUrl, String newsTitle, String newsContent)
{
this.newsIconUrl = newsIconUrl;
this.newsTitle = newsTitle;
this.newsContent = newsContent;
}
public String getNewsIconUrl() {
return newsIconUrl;
}
public void setNewsIconUrl(String newsIconUrl) {
this.newsIconUrl = newsIconUrl;
}
public String getNewsTitle() {
return newsTitle;
}
public void setNewsTitle(String newsTitle) {
this.newsTitle = newsTitle;
}
public String getNewsContent() {
return newsContent;
}
public void setNewsContent(String newsContent) {
this.newsContent = newsContent;
}
}
2 适配器NewsAdapter.java
package com.example.imoocnews; import java.util.List; import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.AbsListView; /**
* listview的适配器，包括上下文对象和数据源
* 提高listview的效率：当listview滚动时不去加载可见项图片，停止滚动后再开始加载
*/
public class NewsAdapter extends BaseAdapter implements AbsListView.OnScrollListener{ private LayoutInflater mInflater;
private List<NewsBean> mList;
private ImageLoader mImageLoader;
private int mStart, mEnd;//listview屏幕可视范围内的第一条item和最后一个item
public static String URLS[];//设置一个数组，用来保存所有图片的URL
private boolean mFirstIn;//判断是否是第一次启动程序 public NewsAdapter(Context context, List<NewsBean> data, ListView listView) {
mInflater = LayoutInflater.from(context);
this.mList = data;
mImageLoader = new ImageLoader(listView);//在这里初始化，能够保证只有一个imageloader的实例，即只有一个LruCache的实例
URLS = new String[data.size()];
for (int i = 0; i < data.size(); i++) {
URLS[i] = data.get(i).getNewsIconUrl();//将data中的每一个URL赋值给数组
}
listView.setOnScrollListener(this);
mFirstIn = true;//写在构造函数中，第一次调用newsAdapter时表示第一次启动程序，显示listview
} @Override
public int getCount() {
return mList.size();
} @Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return mList.get(position);
} @Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return position;
} @Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
if (convertView == null) {
convertView = mInflater.inflate(R.layout.item, null);
holder = new ViewHolder();
holder.ivIcon = (ImageView) convertView.findViewById(R.id.iv_icon);
holder.tvTitle = (TextView) convertView.findViewById(R.id.tv_title);
holder.tvContent = (TextView) convertView.findViewById(R.id.tv_content);
convertView.setTag(holder);
}else {
holder = (ViewHolder) convertView.getTag();
}
//holder.ivIcon.setImageResource(R.drawable.ic_launcher);
String url = mList.get(position).getNewsIconUrl();
holder.ivIcon.setTag(url);//给ImageView设置标志，即对应的图片网址
//利用thread类实现异步加载图片，我们这里将其注释，使用AsyncTask的方式
//new ImageLoader().showImageByThread(holder.ivIcon, mList.get(position).getNewsIconUrl());
mImageLoader.showImages(holder.ivIcon, url); holder.tvTitle.setText(mList.get(position).getNewsTitle());
holder.tvContent.setText(mList.get(position).getNewsContent());
return convertView;
}
class ViewHolder
{
public ImageView ivIcon;
public TextView tvTitle, tvContent;
}
/*
* 当listview滑动的时候调用
*/
@Override
public void onScroll(AbsListView view, int firstVisibleItem,
int visibleItemCount, int totalItemCount) {
mStart = firstVisibleItem;
mEnd = mStart + visibleItemCount;
//只在第一次加载的时候调用
if (mFirstIn && visibleItemCount >0) {//表示第一次加载listview并且已经绘制了可见范围内的item
mImageLoader.loadImages(mStart, mEnd);
mFirstIn = false;//加载图片后即设为false
}
} /*
* 当listview滑动状态变化时调用
*/
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
if (scrollState == SCROLL_STATE_IDLE) {//listview停止滚动
//加载可见项
mImageLoader.loadImages(mStart, mEnd); }else {
//停止加载任务
mImageLoader.cancelAllTasks();
} } }
3 访问图片的实现ImageLoader.java
package com.example.imoocnews; import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.HashSet;
import java.util.Set; import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Message;
import android.util.LruCache;
import android.widget.ImageView;
import android.widget.ListView; public class ImageLoader {
private ImageView mImageView;
private String url;
//当图片加载过后就将图片缓存到本地，下次便不用重新联网获取，直接从本地缓存获取即可,一个图片即string url --> bitmap
private LruCache<String, Bitmap> mCache;
private ListView mListView;
private Set<NewsAsyncTask> mTasks;//从start到end范围每次执行加载图片任务的集合 public ImageLoader(ListView listView)
{
mListView = listView;
mTasks = new HashSet<ImageLoader.NewsAsyncTask>();
int maxMemory = (int) Runtime.getRuntime().maxMemory();//获取最大可用内存
int cacheSize = maxMemory/4;//设置缓存的大小
mCache = new LruCache<String, Bitmap>(cacheSize){
@Override
protected int sizeOf(String key, Bitmap value) {
// 每次将图片存入缓存时返回图片的大小
return value.getByteCount();
}
};
}
/**
* 将已联网获取成功的图片加入到缓存中
* @param bitmap
*/
public void addBitmapToCache(String url, Bitmap bitmap)
{
//在将图片缓存到本地之前要判断这个图片是否已经缓存过了
if (getBitmapFromCache(url) == null) {
mCache.put(url, bitmap);
}
}
/**
* 通过URL从缓存中取出相应的图片
*/
public Bitmap getBitmapFromCache(String url)
{
return mCache.get(url);
} private Handler mHandler = new Handler(){
public void handleMessage(Message msg) {
super.handleMessage(msg);
//通过tag使得ImageView和它对应的URL绑定，这样在上下滑动listview时ImageView显示的图片就始终是正确的
//否则，由于listview的缓存机制，ImageView会先显示出上次加载成功时的图片，然后再显示正确的图片
if (mImageView.getTag().equals(url)) {
mImageView.setImageBitmap((Bitmap) msg.obj);//使用handler在主线程中更新UI，并将URL对应的图片设置给控件imageview
} }
}; /**
* 通过使用Thread的方式从网络上获取图片
*/
public void showImageByThread(ImageView imageView, final String iconUrl)
{
mImageView = imageView;
url = iconUrl;
new Thread(){
@Override
public void run() {
// 在新的进程中实现图片的加载
super.run();
//从url中获得bitmap，将bitmap发送给主线程
Bitmap bitmap = getBitmapFromUrl(iconUrl);
Message message = Message.obtain();
message.obj = bitmap;
mHandler.sendMessage(message);
}
}.start();
}
public Bitmap getBitmapFromUrl(String urlString)
{
InputStream is = null;
Bitmap bitmap;
try {
URL url = new URL(urlString);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
is = new BufferedInputStream(connection.getInputStream());
bitmap = BitmapFactory.decodeStream(is);
connection.disconnect();
//Thread.sleep(1000);
return bitmap;
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally
{
try {
is.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return null;
} /**
* 加载listview可见范围内的所有图片
*
*/
public void loadImages(int start, int end)
{
for (int i = start; i < end; i++) {
String url = NewsAdapter.URLS[i];
//看是否能从缓存中取出对应的图片
Bitmap bitmap = getBitmapFromCache(url);
//如果缓存中没有，就要对每个url执行异步加载任务去获取图片
if (bitmap == null) {
NewsAsyncTask task = new NewsAsyncTask(url);
task.execute(url);
mTasks.add(task); }else {
//如果缓存中存在此图片，直接将其设置给对应的imageview即可
//因为我们之前给imageview设置的tag就是URL,可以利用findViewWithTag直接在这里获取到
ImageView imageView = (ImageView) mListView.findViewWithTag(url);
imageView.setImageBitmap(bitmap);
}
}
}
/**
* 取消所有正在进行的图片加载任务
*/
public void cancelAllTasks()
{
if (mTasks != null) {
for(NewsAsyncTask task : mTasks)
{
task.cancel(false);
}
}
} public void showImages(ImageView imageView, String iconUrl)
{
//是否能从缓存中取出对应的图片
Bitmap bitmap = getBitmapFromCache(iconUrl);
if (bitmap == null) {
imageView.setImageResource(R.drawable.ic_launcher);//显示默认图片
}else {
//如果缓存中存在此图片，直接将其设置给对应的imageview即可
imageView.setImageBitmap(bitmap);
} }
/**
* 使用AsyncTask实现图片的异步加载
*/
class NewsAsyncTask extends AsyncTask<String, Void, Bitmap>
{
//private ImageView mImageView;
private String mUrl;
public NewsAsyncTask(String url) {
// mImageView = image;
mUrl = url;
}
@Override
protected Bitmap doInBackground(String... params) {
String url = params[0];
Bitmap bitmap = getBitmapFromUrl(url);//从网络上得到图片
if (bitmap != null) {
addBitmapToCache(url, bitmap);//获取图片成功将图片存入缓存中
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
// 后台获取图片的任务完成时调用此方法
super.onPostExecute(bitmap);
//给imageview设置图片
ImageView imageView = (ImageView) mListView.findViewWithTag(mUrl);
if (imageView != null && bitmap != null) {
imageView.setImageBitmap(bitmap);
}
mTasks.remove(this);
}
} }
4. MainActivity.java
package com.example.imoocnews; import java.io.BufferedReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.util.ArrayList;
import java.util.List; import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject; import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView; public class MainActivity extends Activity { private ListView mListView;
private static String jsonURL = "http://www.imooc.com/api/teacher?type=4&num=30";//json数据网址 @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListView = (ListView) findViewById(R.id.lv_main);
new NewsAsyncTask().execute(jsonURL);
}
/**
* 将URL网址上的json数据转化为我们所需的newsbean对象
* @return
*/
private List<NewsBean> getJsonData(String url)
{
List<NewsBean> newsBeanList = new ArrayList<NewsBean>();//保存解析出来的所有的数据
try {
//获取到json字符串
String jsonString = readStream(new URL(url).openStream());//和url.openConnection().getInputStream()一样
//Log.d("MainActivity", jsonString);
//将获取到的json字符串变为jsonObject对象，打开网址可以看出data节点是一个jsonArray,array里面存放了一个个的jsonObject
NewsBean newsBean;
JSONObject jsonObject;
String newsUrl = null;
String newsTitle = null;
String newsContent = null;
jsonObject = new JSONObject(jsonString);
JSONArray jsonArray = jsonObject.getJSONArray("data");
for (int i = 0; i < jsonArray.length(); i++) {
jsonObject = jsonArray.getJSONObject(i);
newsUrl = jsonObject.getString("picSmall");//图片网址
newsTitle = jsonObject.getString("name");//title
newsContent = jsonObject.getString("description");//content
newsBean = new NewsBean(newsUrl, newsTitle, newsContent);
newsBeanList.add(newsBean);
}
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (JSONException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return newsBeanList;
} /**
* 解析网络返回的数据
*/
private String readStream(InputStream is)
{
InputStreamReader isReader;
String result = "";
String line = "";
try {
isReader = new InputStreamReader(is, "utf-8");//将字节流转化为字符流
BufferedReader buffReader = new BufferedReader(isReader);//从字符输入流中读取文本，缓冲各个字符，从而实现字符、数组和行的高效读取
while ((line = buffReader.readLine()) != null) {
result += line;
}
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} return result;
}
/**
* 构造一个AsyncTask，传入String类型的URL，返回一个NewsBean对象，每一个对象就是
* listview中的每一行数据，包括一个icon,title,content
*/
class NewsAsyncTask extends AsyncTask<String, Void, List<NewsBean>>
{ @Override
protected List<NewsBean> doInBackground(String... params) {
return getJsonData(params[0]);
} @Override
protected void onPostExecute(List<NewsBean> result) {
super.onPostExecute(result);
// 访问网络并解析json成功后返回结果，即我们设置的List<NewsBean>
NewsAdapter adapter = new NewsAdapter(MainActivity.this, result, mListView);
mListView.setAdapter(adapter);
} } }